This invention relates to highly maneuverable prime movers, particularly those adapted for lifting and transporting objects and ordered arrays of objects in locations where aisle space is limited.
Small-sized prime movers having excellent maneuverability are well known in the prior art. The common fork lift truck is one such example. These normally have rear wheel steering and, because of the very short wheelbase, can readily maneuver in tight spaces. The problem of maneuverability becomes more complex as the size of the equipment increases. The prior art is replete with examples of steering systems designed to improve the maneuverability of various types of vehicles. Two approaches have generally been followed. One has been to provide orthogonal steering capability for the vehicle. Some means is provided for turning all of the wheels 90.degree. from the normal straight-line path. This gives the vehicle the capability of making a zero radius 90.degree. turn. Early examples are seen in the patents to Adams, 1,901,276 and Budd, U.S. No. 1,577,559. In both of these examples the wheels were driven by a conventional mechanical linkage and a large part of the inventor's effort was dedicated to devising a suitable drive means. Other approaches can be typified by those found in the following United States Pat. Nos.: Pellizzetti, 2,863,518; Houlton, 3,197,229; Wilson, 3,825,087; Norris, 3,972,379; and Larson, Can. 1,010,373. Each of these vehicles is adapted to have orthogonal steering capability. The wheels are driven with individual hydraulic motors, a practice which greatly simplifies construction over the two examples cited earlier. The patent to Larson contains on excellent review of various types of steering systems.
Another approach to improving maneuverability has been to provide four-wheel steering. Normally, in vehicles so equipped, the front wheels or rear wheels may be steered independently. In addition, they may be steered simultaneously to provide one or both of two steering modes. Where, for example, the front wheels are turned to the right, the rear wheels may be appropriately turned to the left. This procedure significantly reduces the turning radius of the vehicle. In the other mode, both the front wheels and the rear wheels are turned in the same direction. This enables oblique or crab-wise travel. Examples of prime movers having this type of steering can be seen in U.S. patents to Conrad, U.S. Pat. No. 3,724,585, and Queen, U.S. Pat. No. 3,856,102. In addition, some of the earlier noted vehicles having orthogonal capability have also been designed to have four-wheel steering.
None of the prior art devices havng orthogonal steering capability have had Ackermann steering systems. In order to avoid wheel scuffing or dragging when turning a corner, the projections of the front wheel axles must meet at a common point on a vertical plane projected through the rear wheel axle. This requires the inside wheel in a turn to be at a sharper angle than the outside wheel. The differential between the wheels varies as the vehicle enters and withdraws from a turn. Linkages which achieve this requirement are termed "Ackermann steering systems". Ackermann systems are in almost universal use on all highway adapted vehicles.
The fact that the earlier vehicles having orthogonal capability do not have Ackermann steering systems poses a major disadvantage. For one thing, tire wear is greatly increased. A second problem is that such vehicles are unpleasant to operate. As they turn, the inside wheel in particular tends to bounce or vibrate as its scuffs across the ground surface. It can only be surmised that the complexities required for shifting from a normal to an orthogonal steering mode precluded provision of an Ackermann steering system in prior art highly maneuverable prime movers.
It might also be noted that the prior art shows prime movers having relatively versatile steering systems which also have rotatable operator cabs and load handling adaptation. One such system is shown in the aforementioned patent to Queen, U.S. Pat. No. 3,856,102. Another vehicle adapted for logging is shown in U.S. Pat. No. 3,981,336 to Levesque where the prime mover travels on four tracks and has a rotatable turret containing a boom equipped with a log-processing unit. In the case of the Levesque device the prime mover is skid-steered; i.e., the tracks on one side are braked while those on the other side are driven normally. The vehicle is thus caused to turn toward the side on which the drive elements are braked.
To the present inventors' knowledge there has been no prime mover heretofore available which combined a true Ackermann steering system with orthogonal steering capability.